build_image: silence the "No space left on device" message
build_image: silence the "No space left on device" message

BUG=chromium-os:12219
TEST=run it, see the message gone

Change-Id: I54174c50dd38e74989e7d63a798dc7f9ea011933

R=thieule@chromium.org

Review URL: http://codereview.chromium.org/6696095

crawford/scripts,sigma/coreos-scripts,marineam/coreos-scripts,bcwaldon/coreos-scripts,BugRoger/scripts,mischief/scripts,crawford/scripts,glevand/coreos--scripts,BugRoger/scripts,andrejro/coreos--scripts,trnubo/scripts,endocode/scripts,nicescale/scripts,smilart/scripts,endocode/scripts,mischief/scripts,glevand/coreos--scripts,mjg59/scripts,gtank/scripts,vmware/coreos-scripts,gtank/scripts,zhang0137/scripts,andrejro/coreos--scripts,marineam/coreos-scripts,mjg59/scripts,smilart/scripts,BugRoger/scripts,sigma/coreos-scripts,fivethreeo/scripts,endocode/scripts,vmware/coreos-scripts,bcwaldon/coreos-scripts,smilart/scripts,gtank/scripts,nicescale/scripts,mjg59/scripts,zhang0137/scripts,marineam/coreos-scripts,mischief/scripts,vmware/coreos-scripts,sigma/coreos-scripts,coreos/scripts,trnubo/scripts,fivethreeo/scripts,andrejro/coreos--scripts,coreos/scripts,bcwaldon/coreos-scripts,trnubo/scripts,nicescale/scripts,crawford/scripts,zhang0137/scripts,fivethreeo/scripts